#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include "b64.h"

#ifdef b64_USE_CUSTOM_MALLOC
extern void* b64_malloc(size_t);
#endif

#ifdef b64_USE_CUSTOM_REALLOC
extern void* b64_realloc(void*, size_t);
#endif

unsigned char *
b64_decode(const char *src, size_t len)
{
	return b64_decode_ex(src, len, NULL);
}

unsigned char *
b64_decode_ex(const char *src, size_t len, size_t *decsize)
{
	int i = 0;
	int j = 0;
	int l = 0;
	size_t size = 0;
	unsigned char *dec = NULL;
	unsigned char buf[3];
	unsigned char tmp[4];

	// alloc
	dec = (unsigned char *) b64_malloc(1);
	if (NULL == dec)
	{
		return NULL;
	}

	// parse until end of source
	while (len--)
	{
		// break if char is `=' or not base64 char
		if ('=' == src[j])
		{
			break;
		}
		if (!(isalnum(src[j]) || '+' == src[j] || '/' == src[j]))
		{
			break;
		}

		// read up to 4 bytes at a time into `tmp'
		tmp[i++] = src[j++];

		// if 4 bytes read then decode into `buf'
		if (4 == i)
		{
			// translate values in `tmp' from table
			for (i = 0; i < 4; ++i)
			{
				// find translation char in `b64_table'
				for (l = 0; l < 64; ++l)
				{
					if (tmp[i] == b64_table[l])
					{
						tmp[i] = l;
						break;
					}
				}
			}

			// decode
			buf[0] = (tmp[0] << 2) + ((tmp[1] & 0x30) >> 4);
			buf[1] = ((tmp[1] & 0xf) << 4) + ((tmp[2] & 0x3c) >> 2);
			buf[2] = ((tmp[2] & 0x3) << 6) + tmp[3];

			// write decoded buffer to `dec'
			dec = (unsigned char *) b64_realloc(dec, size + 3);
			if (dec != NULL)
			{
				for (i = 0; i < 3; ++i)
				{
					dec[size++] = buf[i];
				}
			}
			else
			{
				return NULL;
			}

			// reset
			i = 0;
		}
	}

	// remainder
	if (i > 0)
	{
		// fill `tmp' with `\0' at most 4 times
		for (j = i; j < 4; ++j)
		{
			tmp[j] = '\0';
		}

		// translate remainder
		for (j = 0; j < 4; ++j)
		{
			// find translation char in `b64_table'
			for (l = 0; l < 64; ++l)
			{
				if (tmp[j] == b64_table[l])
				{
					tmp[j] = l;
					break;
				}
			}
		}

		// decode remainder
		buf[0] = (tmp[0] << 2) + ((tmp[1] & 0x30) >> 4);
		buf[1] = ((tmp[1] & 0xf) << 4) + ((tmp[2] & 0x3c) >> 2);
		buf[2] = ((tmp[2] & 0x3) << 6) + tmp[3];

		// write remainer decoded buffer to `dec'
		dec = (unsigned char *) b64_realloc(dec, size + (i - 1));
		if (dec != NULL)
		{
			for (j = 0; (j < i - 1); ++j)
			{
				dec[size++] = buf[j];
			}
		}
		else
		{
			return NULL;
		}
	}

	// Make sure we have enough space to add '\0' character at end.
	dec = (unsigned char *) b64_realloc(dec, size + 1);
	if (dec != NULL)
	{
		dec[size] = '\0';
	}
	else
	{
		return NULL;
	}

	// Return back the size of decoded string if demanded.
	if (decsize != NULL)
	{
		*decsize = size;
	}

	return dec;
}
